1. Field of the Invention
This invention relates to pedagogic apparatus adapted for demonstration in the theory of heat sealing and the operation and use of thermal pulse sealing machines.
2. Brief Description of the Prior Art
Plastic sealing through various welding techniques has traditionally been accomplished with the application of heat and pressure. The heat has been generated by various sources such as infrared radiation, dielectric, ultrasonic, and electrical resistance, while pressure is required to force the heated surfaces together to achieve the bond.
The term impulse sealing as been used to designate the technique of bonding or welding relatively thin films utilizing pressure and a very rapid heat pulse generated by a resistance element. The films were then cooled while maintaining compressive pressure. Various thermoplastic films which have been bonded utilizing impulse sealing include polystyrene, polyethylene, polypropylene, polyvinyl chloride and the like.
Operating demonstration equipment has been desirable when instructing students in the theory and practice of plastic fabrication technology. Although commercial production machinery have been used for demonstration, several disadvantages have been encountered with the use of such equipment in the primary and secondary school pedagogic environment wherein the machinery are operated by unfamiliar students as part of their course of study.
Commercial impulse sealers were generally designed for application in seaming thin films and characteristically employed an extremely short heat cycle generated by a short impulse of current flow through a resistance ribbon. The rapid heat cycle provided inherent limitations on heat generating capacity, thus restricting the thickness of the layers to be bonded. Additionally, the short current impulse (generally not longer than 2 seconds) inhibited student perception and cognizance of the precise heating cycle.
Production impulse sealers additionally lacked the versatility to perform satisfactorily in a pedagogic environment wherein multiple custom projects were constructed. Typically, prior sealers were provided with particular jaw lengths usually not appreciably greater than the dimension of the films to be sealed in production. In the school shop, however, student projects were quite diversified and while one student's project was the construction of a small article, e.g. a billfold, another student's project might very well have been a larger article, e.g. a tote bag, or one wherein thick layers of thermoplastic materials were to be bonded.
When relatively thick layers, i.e. over 10 mils total thickness, of thermoplastic material were bonded, the maximum heat generated in a single impulse cycle of a commercial impulse sealer was insufficient. Attempts to avoid this limitation included multiple rapid recycling to generate sufficient heat. With the layers of work pieces absorbing the heat generated, heat was not effectively dissipated in areas of the jaws not contacted by the work pieces. High temperature areas resulted in causing severe burning or scorching of protective sheaths which covered the jaws.